if you have a trust between an MIT kerberos realm and an Active Directory Domain, say EXAMPLE.COM and AD.EXAMPLE.COM it will always guess that the server principle is server$@EXAMPLE.COM and never try server$@AD.EXAMPLE.COM
so, if do a kinit for user@EXAMPLE.COM then try to do a smbclient -k //server.ad.example.com/whatever it will end up trying the principal server$@EXAMPLE.COM instead of server$@AD.EXAMPLE.COM